This invention relates to a hydraulic valve lifter mechanism which constitutes part of a valve train of an internal combustion engine, particularly, on an automobile.
Traditionally, intake and exhaust valves of an internal combustion engine, particularly an automotive engine, are made to open and close each with a fixed amount of valve lift determined by giving priority to the maximum output of the engine. However, the amount of valve lift thus determined is too large to maintain a desirable valve overlap period at low engine speeds or during idling. An excessively large valve overlap under such conditions tends to cause unfavorable phenomena such as less efficient combustion by reason of backward flow of the combustion gas.
To solve this problem, it has been proposed to include in the valve train a hydraulic valve lifter or tappet which has the function of varying the valve opening and closing timing and the amount of valve lift depending on the pressure of engine lubricating oil as an indication of the engine speed. A typical example of such hydraulic valve lifters is disclosed in U.S. Pat. No. 4,020,806 to Aoyama et al. This valve lifter has a cylindrical body slidably received in a bore formed in an engine block such that a closed end of the cylinder keeps in contact with a cam mounted on the camshaft. A plunger connected to a push rod is slidably received in the body to define an oil pressure chamber in the body, and a free piston is disposed in this chamber under the force of a compression spring such that the volume of the oil pressure chamber varies depending on the oil pressure applied to this chamber. When the body is lifted by the cam lobe while oil pressure applied to the oil pressure chamber is below a certain level, the movement of the body relative to the plunger causes an abrupt increase in the oil pressure in the chamber and hence an abrupt movement of the free piston to increase the volume of the chamber. As the result, the plunger moves towards the closed end of the body whereby the action of the valve begins at retarded timing and the amount of valve lift is decreased.
However, it becomes a disadvantage of this valve lifter that under low engine speed conditions the linkage including the valve and the push rod is forced to begin its action abruptly against the inertia of the respective components since a great impulsive force is exerted on each component at the start of its action and, as a consequence, the action of the linkage produces a loud noise. Besides, the impulsive manner of actuation of the linkage is unfavorable to the durability of the respective components.